Impact of aerosol and water vapour on SW radiation at the surface: Sensitivity study and applications

Highlights

• Impact of aerosols and water vapour on downwelling SW irradiance was studied.

• Two models to estimate this impact on downwelling SW irradiance were proposed.

• Model estimations were compared to real measurements registered at five stations.

• Both models show good performance with differences under 3% in 86% of cases.

Abstract

This study provides a sensitivity analysis of the downwelling shortwave (SW) irradiance at the Earth's surface with respect to the aerosols and the water vapour. Aerosols are characterized by optical thickness (AOD) and single scattering albedo (SSA), while water vapour is quantified by precipitable water vapour (PWV). For this purpose, downwelling SW irradiances under cloud-free conditions and a wide range of AOD, SSA and PWV values are simulated using libRadtran radiative transfer code. Results show SW irradiance decreases as AOD or PWV increases or as SSA decreases. The general analysis also shows that the combined effect of these quantities is not just the sum of the individual effects. Based on the results, two models are proposed to estimate the effect of aerosols and water vapour on SW irradiance. The two models are validated through comparing their results with radiation measurements registered at nine stations around the world. Models using two and three quantities show differences lower than 3% in 84% and 88% of cases, respectively. This good performance indicates the reliability of the models to estimate the effect of aerosols and water vapour on SW irradiance when no radiation measurements exist, provided that aerosol and water vapour information is available.

Introduction

Solar radiation is the main energy source for the Earth–atmosphere system. It is well known that changes in the atmospheric composition could significantly affect the radiative budget and, as a result, the global temperature of the Earth (Forster et al., 2007). Two of the most important components of this system affecting the atmospheric transfer of solar radiation toward the Earth's surface are, for cloud-free conditions, aerosols and water vapour. Aerosols are atmospheric particles, of natural and anthropogenic origin, playing an important role in the climate system due to their (direct and indirect) effects on the Earth's radiative balance (Boucher et al., 2013). On the other hand, water vapour is an important gas, since it induces a surface warming through the trapping of longwave radiation (greenhouse effect) and a surface cooling by the absorption of shortwave radiation. Water vapour strongly absorbs in the spectral region beyond 0.5 μm of the solar spectrum, in the visible and the infrared solar wavelengths (Liou, 1980).

Aerosol direct effects have been thoroughly studied in the shortwave (SW) spectral range, which is the portion of the solar spectrum that extends from approximately 280 to 2800 nm. Many studies performed to analyse the effect of aerosols on downwelling SW irradiance at the surface are based on the calculation of the aerosol radiative forcing (ARF) (e.g., Costa et al., 2006; Di Sarra et al., 2008; Santos et al., 2008; Tzanis and Varotsos, 2008; Di Biagio et al., 2009; García et al., 2012; Santos et al., 2013; Esteve et al., 2014; Mateos et al., 2014; Che et al., 2015; Obregón et al., 2015a; Dhar et al., 2017; Fernández et al., 2017; Obregón et al., 2017; Mai, B.R., and Coauthors, 2018; Péré et al., 2018). However, the effect of water vapour on the downwelling SW radiation is scarcely studied (e.g., Mateos et al., 2013; Obregón et al., 2015b; Vaquero-Martínez et al., 2018). It should also be mentioned that in most radiative forcing studies, usually the effect of the whole set of aerosols or water vapour is analysed but not their individual effects. Only some of them analyse the effect of individual aerosol quantities on the ARF. For example, Di Biagio et al. (2009) and Mateos et al. (2014) have investigated the effects of the single scattering albedo (SSA) on ARF, concluding that absorbing aerosols lead to more negative ARF values.

The effect of water vapour and aerosols on downwelling SW irradiance is scarcely analysed (e.g., Bilbao et al., 2014; Trabelsi et al., 2015). In addition, these studies are based on measurements and, therefore, limited to specific periods, events or experimental campaigns, and restricted to the specific aerosol and water vapour values of the study areas. Thus, for example, Bilbao et al. (2014) analysed the effects of individual aerosol and water vapour quantities on the SW irradiance. Specifically, this study addresses the quantification of the relative change of SW irradiance measured in Marsaxlokk, Malta, for different aerosol optical depth (AOD) and precipitable water vapour (PWV) values registered in that station during a measurement campaign between May and October 2012. Only 47 days were used. During this campaign, the AOD values at 550 nm does not exceed 0.6, with the majority of the cases in the range 0–0.3, and the majority of the PWV values in the range 20–40 mm. The validity of the results is limited to the campaign and, as a result, not of general use at other sites. This is the main limitation of this type of studies, especially when the sensitivity of a given quantity to different variables is the aim of the analysis. Therefore, modelling appears as a good alternative to analyse a variety of atmospheric situations (such as different aerosol and water vapour conditions), which may not be registered during an intensive campaign or at a specific location, and provided that the model results can be trusted. A good example of using modelling for this purpose is the work performed by Di Biagio et al. (2012) for the Arctic. In this work, we propose a similar approach but generalizing for different latitudes, water vapour and aerosols conditions.

To our knowledge, there are no studies that quantify in a general way the effect of aerosols and water vapour on the solar radiation at the surface. The main objective of this study is twofold: 1) analyse and quantify the sensitivity of the downwelling SW irradiance at the Earth's surface with respect to AOD, SSA and PWV, and 2) from the results of the sensitivity analysis, propose two models to estimate the effect of aerosols and water vapour on SW irradiance and validate through comparing with radiation measurements. Among the factors involved in the sensitivity analysis, AOD and SSA are used to describe the aerosol conditions and PWV quantifies the water vapour amount. This validation will allow proposing an empirical relationship to estimate the aerosol and water vapour effects on SW radiation based on AOD, SSA and PWV data, when no radiation measurements are available. This can be important for climate, air quality or energy applications. Most studies proposing empirical or semi-empirical models or equations are focused on estimating surface radiation from satellite data or models (e.g., Janjai et al., 2013; Lefèvre et al., 2013; Laaroussi et al., 2016), but few propose these models or equations to calculate effects on SW radiation (e.g. Xia et al., 2007; Stone et al., 2011). So, for example, Xia et al. (2007) developed empirical equations to describe AOD effects on surface irradiance and Stone et al. (2011) proposed empirical determinations of the longwave and shortwave radiative forcing efficiencies of wildfire smoke.

For this purpose, downwelling SW irradiance values in cloud-free conditions have been simulated with a radiative transfer model for a wide range of values of each quantity. The use of simulations also allows analysing the direct effect of each quantity on the downwelling SW irradiance, as well as their combined effect, which according to Bhatia et al. (2015) is not usually analysed therefore worth to be investigated. Also this combined effect upon the SW irradiance would not be possible if SW measurements were used instead of simulated SW irradiance, since those already incorporate the joint effect of the above mentioned quantities. Moreover, the quantification of the effects of these atmospheric constituents on the SW irradiance is not limited to the period with available measurements, and it is neither limited to the availability of radiation measurements under clear sky conditions, less frequent at certain times of the year. For these reasons, this study has a greater potential of generalization compared with other studies (e.g., Di Biagio et al., 2012) and contributes to improve the understanding and quantification of the influence of these atmospheric constituents on the downwelling SW irradiance at the Earth's surface.

The paper is structured as follows: the materials and methods are presented in Section 2. Section 3 shows the principal results. In Subsection 3.1 are shown the results regarding the effects of aerosols and water vapour on the SW irradiance, both the individual effect of each quantity as well as the combined effects of the three quantities. In Subsection 3.2, the validation of the models proposed is presented. Finally, the main conclusions are summarized in Section 4.